Dynamic Modelling Of Offshore Wind Turbines Subjected To Earthquake Loadings And The Corresponding Mitigations

Earthquake has been a significant factor against the development of wind energy for offshore areas including Southern-East of China.Relevant international standards and guidelines for wind turbine design suggest to refer building and constrction codes regarding earthquake hazards.The coupled effect between aerodynamic loads and seismic load is fully ignored.This assumption may be invalid for offshore wind turbines with large rotors.In order to gurrentee the integrality and safety of wind turbines located at earthquake-prone areas,it is imperative to investigate the structural dynamic response characteristics of wind turbines subjected to multiple loadings consisting of wind,wave,current and earthquake,by taking into account of Soil Structure Interaction（SSI）effects.There is also a need to apply effective structural control approaches for vibration mitigation with referring to the characteristics of earthquake-induced responses.In order to address the limitations of seismic analysis of wind turbines as stated previously,this thesis aims to present a generic Seismic Analysis Framework（SAF）by developing and solving the fully-coupled aero-hydro-servo-structural-seismic model of offshore wind turbines.For the improvement of relevant international standards and guidelines regarding wind turbine load demand under earthquake loading conditions,the sensitivity of structural responses to earthquake intensity and aerodynamic damping has been investigated.On the basis of the obtained results,a nonlinear model for load demand estimation is developed.In addition,SSI effects of the NREL 5.0 MW wind turbine are modelled using Apparent Fixity（AF）,Coupled Springs（CS）and Distributed Springs（DS）approaches.The earthquake-induced vibrations influenced by linear and nonlinear SSI effects have been investigated.Moreover,this thesis presents comparisons between the seismic behaviors of onshore and offshore DTU 10.0 MW wind turbines under multiple loadings including wind and earthquake.The effect of Tuned Mass Damper（TMD）for mitigating dynamic responses of wind turbines under wind and earthquake loadings has been examined.The research contents and conclusions are presented as follows:1.By taking into account the combination effect of turbulent wind,irregular wave,unsteady current and random earthquake loadings,a fully-coupled aero-hydro-servo-elastic-seismic model is developed.The SAF for seismic analysis of offshore wind turbines is developed by implementing the fully-coupled model.The reliability,effectiveness and accuracy of SAF have been validated by comparisons against GH Bladed and NREL Seismic.SAF is more superior to GH Bladed with respect to adjusting frequency contents of the input ground motion in order to match a specific target response spectrum.Moreover,SAF could take into account the vertical component of earthquakes as well.In addition,SAF is more generic compared to NREL Seismic since the selection of experience parameters has been avoided for the earthquake load calculation.2.The sensitivity analysis of structural responses of wind turbines under different operational states to the earthquake intensity has been performed.A sensitivity coefficient is obtained by fitting the peak tower-top displacements corresponding to earthquake intensity.A model for estimating tower-top acceleration and a nonlinear for predicting tower bending moment are developed,respectively.The comparison against relevant standards and reference publications indicates that the model developed in this thesis is more accurate for predicting tower bending moment demands.3.The SSI model of NREL 5.0 MW wind turbine has been modelled using AF,CS and DS methods,respectively.The seismic behaviors of the wind turbine under 28 ground motions combined with wind,wave and current have been obtained.The results indicate that AF model can reflect realistic situation between compared to CS model.Compared to rigid foundation model,the nonlinear SSI model has larger vibration contributed by the 1^st order mode.The amplitude corresponding to the 1^st order natural frequency of the support structure is 4 times of that with a rigid foundation model.Moreover,neglecting the SSI effect could result in misjudgment on the consequence from an emergency shutdown.4.On the basis of DTU 10.0 MW wind turbine model,the monopile that was designed for 50 m depth areas is improved.A variable-speed and pitch-to-feather controller for the 10 MW wind turbine is developed.The tower-top displacements and tower-base bending moments of the onshore and offshore DTU 10.0 MW wind turbines under the excitation of 28 earthquakes are calculated with the use of SAF.The results indicate that turbulent wind could reduce the variation range of tower-top displacement by 40%above.Meanwhile,earthquake loading dominates tower-base bending moment.An approximate linear increase trend of the peak tower-top displacement against Pseudo Spectral Acceleration（PSA）is observed for both onshore and offshore wind turbines.It is noted the coupled loading case has smaller increase rate than that of earthquake-only case.This indicates that turbulent wind has a positive effect on moderating the increase of earthquake-induced responses by dissipating energy from the earthquake excitation.It is notable this moderation effect is more obvious for the onshore wind turbine.In addition,the tower-base bending moment of the offshore wind turbine is dominated by earthquake loading.Compared to the wind-only case,earthquake-only case has 6 times larger peak tower-base bending moment.5.In order to mitigate consequence to earthquake events,TMD is employed for structural control.The SAFISC（Seismic Analysis Framework Including Structural Control）is developed by implementing structural control capability based on SAF.The sensitivity analysis of control parameters including tuned frequency ratio,damping ratio and mass ratio has been performed for mitigating structural dynamic responses.Nonlinear response surface models associated with mitigation effect and control parameters are presented to obtain the optimal control parameters.The application effects of the optimal TMD corresponding to the onshore and offshore wind turbines have been investigated and disscuessed.The results indicate that TMD is effective on mitigating earthquake-induced response.The TMD with optimal tuned frequency and damping could reduce peak of tower-top displacement by 30%above for the onshore wind turbine.With regards to the offshore wind turbine,TMD has a signifincant effect against earthquake consequence.The tower-top vibration could be moderated by 15%for most earthquake excitations with the use of TMD.